Running

Running certainly isn't for everybody. It's both high-intensity -- maybe too high for beginners -- and high-impact. If you can do it, though, the American Council on Exercise reports that you stand to burn up to 17 calories per minute -- that's a formidable 1,020 per hour -- depending on your body weight.

Walking

If you're not up to running, try walking instead. You can do it almost anytime, anywhere, and you don't have to pay for a gym membership. The American Council on Exercise pegs walking at a respectable 9.7 calories per minute, or 582 per hour, again depending on your body weight.

Kettlebells

Cardio exercises are typically the best calorie burners, but an intense kettlebell workout provides the best of both cardio and strength training. An American Council on Exercise-sponsored study pegs the calorie burn at a whopping 20.2 per minute. That'd be more than 1,200 per hour, although it's not reasonable to maintain that level of intensity for that long.

Cycling

According to the Harvard Health Letter, the best calorie burner in the gym is a vigorous ride on a stationary cycle -- although, of course, you can get similar results by biking outside. You stand to burn more than 900 calories per hour, depending on body weight.

Swimming

Not only is swimming an excellent calorie burner -- more than 600 calories per hour per MayoClinic.com -- it's also very easy on your body, especially your joints.

Jumping Rope

If you need an inexpensive, effective weight-loss workout that you can do at home, try jumping rope. According to MayoClinic.com you could stand to burn more than 1,200 calories per hour, depending on your body weight.

Step Aerobics

If you're into step aerobics, the Harvard Health Letter reports that opting for a high-intensity, high-impact class can burn almost 900 calories per hour. Not bad for a workout that uses nothing but your body and a step.

Aerobic Dancing

Aerobic dancing isn't just fun, it's also a great calorie burner. According to the American Council on Exercise, you can burn more than 11 calories per minute -- 660 per hour -- while getting your groove on.

Martial Arts

Taekwondo -- and, by extension, other high-intensity martial arts -- are surprisingly good calorie burners: more than 1,100 per hour, according to MayoClinic.com.

Cross-Country Skiing

Even if it's cold and snowing out, you can still burn a lot of calories outdoors. According to the American Council on Exercise, cross-country skiing can burn more than 11 calories per minute, or 660 per hour, which puts it firmly in the realm of "best exercises for slimming down."